The invention relates to a limit switching apparatus for limiting the movement of a motorized focusing drive, in particular for a microscope having a movable specimen stage.
Reliable specimen/objective protection in microscopy is intended to ensure that the specimen does not come into contact with the chosen objective during focusing, and consequently neither the specimen nor the objective can be damaged.
This problem is solved in the case of manually operating focusing drives by mechanical stops, which block the focusing drive in a certain position, being provided on the microscope stage. Furthermore, mechanical settable locked positions are known, by means of which a set specimen stage position can be fixed reproducibly.
Modern microscope drives have a motorized focusing drive and are equipped with electrical/electronic control devices, by means of which the focus position can be approached and can be stored.
The microscope systems equipped with motorized focusing drives are increasingly used for production inspections, for example in the semiconductor industry. Here it has been found that, under certain circumstances, electronic controls operate unreliably and essential specimen protection cannot be reliably ensured by electronic means.
Furthermore, practice has shown that in production inspections the specimens to be investigated are affected by manufacturing tolerances. For example, microscopes equipped with electronic or mechanical specimen protection cannot be focused sufficiently accurately if the specimen thickness varies. Due to the activated specimen protection, the specimen stage cannot be moved out of the stored focus position closer to the objective and consequently cannot be focused. This has been found to be disadvantageous for production inspections since, for example, in the semiconductor industry the wafers to be investigated may be affected by tolerances of about .+-.20 .mu.m. For the exact focusing of such a specimen, the activated specimen protection must first of all be laboriously switched off and then readjusted.
It is therefore an object of the present invention to improve a limit switching apparatus on a motorized focusing drive for a microscope in such a way that, in addition to the specimen/objective protection, a defined further focusing beyond the fixed focus point is permitted.